irqchip/xtensa-pic: Include header for xtensa_pic_init_legacy()

The declaration for this function is not included, which leads to a harmless warning:

drivers/irqchip/irq-xtensa-pic.c:91:12: error: no previous prototype for 'xtensa_pic_init_legacy' [-Werror=missing-prototypes]

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20230810123404.1222332-1-arnd@kernel.org